Tumblr functions quite differently to Twitter; my take on it is that Twitter is more or less essential for certain types of communication, and it’s where celebrities broadcast their voices. Tumblr, in contrast, appeals to niche groups and is good for posting long-form mixed media messages.

As @zenvelo said, I don’t think you can extrapolate trends from Twitter to Tumblr or vice versa. However, these kinds of numbers can surely be checked, if you’re interested in looking, When Yahoo bought Tumblr a few years ago, there were concerns that they would break it or shut it down, and although they did break a few features, I don’t see a large negative effect on the userbase – but, of course, I see only my corner of my niche group.

Websites only really have a future if they can make money. Sad but true. Server space, bandwidth, upkeep etc cost money and the more users to have the more they cost. Twitter is screwed because it’s having difuculty making money. It’s a cool idea but there’s no real space for adverts and no one wants to pay for an account. Tumblr at least has ads. I know… you hate them, I hate them, everyone hates them…. but users get a share of the revenue and tumblr gets to make some cash.

According to this analysis by Alexa Tumblr, was doing fine until this month. They were cruising along as the 48th most popular internet site in spite of the fact that they their upload/download times are slower than 90% of their competitors. They hit a huge peak in March, then traffic slowed to a two year low in April. But one bad month does not make a trend.

While there’s no foolproof way of tracking a site’s userbase, Tumblr’s Alexa rating is still pretty high (48th globally, 19th in the US), though it is down seven ranks in the past few months. That might seem bad compared to Twitter, which has an even higher Alex rating and less recent decline, but Tumblr usage tends to go up and down, while Twitter usage tends to be steadier (so decline is more of an issue). And as @Lightlyseared points out, the real issue is revenue. Being ranked 48th globally and 19th in the US out of 30 million sites tracked is good no matter how you slice it. So the issue isn’t the userbase, it’s whether or not the site has a sustainable business plan.

I see tumblr like a fused version of Twitter and Instagram, with a capacity for small scale movie clips thrown in. Tumblr can continue to be a social media site that is a jack of all trades, or it can separate its functions and focus on just one. If that is what tumblr decides, I see it becoming an alternative to Twitter. Tumblr could choose to expand on video making hosting capacity and be an alternative to YouTube. If it goes that route it can either present itself as a less “censorship policed” video hosting site then YouTube of it can just throw in some bells and whistles to make it look different from YouTube. It depends on how much controlled interests have in influencing Tumblr. For example,Facebook and Google-based social media (YouTube) platforms are, in my opinion, are just semi privatizef government agencies.